1. Field of the Invention
This invention relates to electrophotography and in particular to photoconductive insulating compositions and elements, and to processes using such compositions and elements.
2. Description of the Prior Art
The process of xerography, as disclosed by Carlson in U.S. Pat. No. 2,297,691, employs an electrophotographic element comprising a support material bearing a coating of an insulating material whose electrical resistance varies with the amount of incident electromagentic radiation it receives, such as during an image-wise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge, generally in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of this surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge of electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or discharge pattern as desired. Deposited marking material can then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor or the like, or transferred to a second element to which it can similarly be fixed. Likewise, the electrostatic charge pattern can be transferred to a second element and developed there.
Various photoconductive insulating materials have been employed in the manufacture of electrophotographic elements. For example, vapors of selenium and vapors of selenium alloys deposited on a suitable support and particles of photoconductive zinc oxide held in a resinous, film-forming binder bave found wide application in present-day document-copying processes.
Since the introduction of electrophotography, a great many organic compounds have also been screened for their photoconductive properties. As a result, a very large number of organic compounds have been known to possess some degree of photoconductivity. Many organic compounds have revealed a useful level of photoconduction and have been incorporated into photoconductive compositions. Among these organic photoconductors are certain of the triphenylamines as described in U.S. Pat. No. 3,180,730 issued Apr. 27, 1965, and the polyarylalkane compounds such as those described in U.S. Pat. No. 3,274,000 issued Sept. 20, 1966; U.S. Pat. No. 3,542,547 issued Nov. 24, 1974; and in U.S. Pat. No. 3,615,402 issued Oct. 26, 1971. Other useful photoconductors include high speed "heterogeneous" or "aggregate" multiphase photoconductive compositions, such as those described in U.S. Pat. No. 3,615,414 issued Oct. 26, 1971.
It is often desirable to modify the imaging parameters of photoconductive insulating compositions. One such parameter is image contrast, which refers broadly to the relationship between the intensity of the electrophotographic image (i.e. its charge magnitude or optical density, if developed) and the exposure range typically used to promote imagewise discriminations. High contrast photoconductive compositions generally produce more substantial variations in charge or optical density over a particular range of imaging exposure intensity than do low contrast compositions. Depending on use, it can be desirable to have a composition that exhibits either high or low contrast imaging characteristics. For example, when reproducing continuous tone images, lower contrast compositions are usually preferred because they have greater ability to reproduce subtle gradations of image detail. However, in some cases higher contrast may be desired to provide a copy that accentuates detail within a particular density range of the original or any portion of the original. Such copies can be useful for interpretation of aerial photographs.